1. Field of the Invention
The present invention relates to wireless communication systems and, more particularly, to cellular communications systems which use an analog control channel (ACCH) and a digital control channel (DCCH) for providing service to a plurality of mobile stations that are capable of operating on the ACCH and/or the DCCH.
2. Related Prior Art Systems
Cellular mobile telephony is one of the fastest growing segments in the worldwide telecommunications market. In the United States, cellular radio systems have been operating since the early 1980s and their subscriber base has steadily increased during this period. Between 1984 and 1992, for example, the number of mobile telephone subscribers in the United States grew from around 25,000 to over 10 million. It is estimated that the number of subscribers will rise to nearly 22 million by year end 1995 and to 90 million by the year 2000.
Cellular telephone service operates much like the fixed, wireline telephone service in homes and offices, except that radio frequencies rather than telephone wires are used to connect telephone calls to and from the mobile subscribers. Each mobile subscriber is assigned a private (10 digit) directory telephone number and is billed based on the amount of "airtime" he or she spends talking on the cellular telephone each month. Many of the service features available to landline telephone users (e.g., call waiting, call forwarding, three-way calling, etc.) are also generally available to mobile subscribers.
In the United States, cellular licenses have been awarded by the Federal Communications Commission (FCC) pursuant to a licensing scheme which divided the country into geographic service markets defined according to the 1980 Census. The major metropolitan markets are called metropolitan statistical areas (MSAs) while the smaller rural markets are called rural statistical areas (RSAs). Only two cellular licenses are awarded for each market. The two cellular systems in each market are commonly referred to as the "A" system and "B" system, respectively. Each of the two systems is allocated a different frequency block in the 800 MHz band (called the A-band and B-band, respectively). Mobile subscribers have the freedom to subscribe to service from either the A-system or the B-system operator (or both). The local system from which service is subscribed is called the "home" system. When travelling ("roaming") outside the home system, a mobile subscriber may be able to obtain service in a distant system if there is a roaming agreement between the operators of the home and "visited" systems.
In a typical cellular radio system as shown in FIG. 1, a geographical area (e.g., a metropolitan area) is divided into several smaller, contiguous radio coverage areas (called "cells") such as cells C1-C10. The cells C1-C10 are served by a corresponding group of fixed radio stations (called "base stations") B1-B10, each of which operates on a subset of the radio frequency (RF) channels assigned to the system. For illustration purposes, the base stations B1-B10 are shown in FIG. 1 to be located at the center of the cells C1-C10, respectively, and are shown to be equipped with omni-directional antennas transmitting equally in all directions. However, the base stations B1-B10 may also be located near the periphery or otherwise away from the centers of the cells C1-C10, and may illuminate the cells C1-C10 with radio signals directionally (e.g., a base station may be equipped with three directional antennas each covering a 120 degrees sector).
The RF channels allocated to any given cell (or sector) may be reallocated to a distant cell in accordance with a frequency reuse pattern as is well known in the art. In each cell (or sector), at least one RF channel (called the "control" or "paging/access" channel) is used to carry control or supervisory messages, and the other RF channels (called the "voice" or "speech" channels) are used to carry voice conversations. The cellular telephone users (mobile subscribers) in the cells C1-C10 are provided with portable (hand-held), transportable (hand-carried) or mobile (car-mounted) telephone units (mobile stations) such as mobile stations M1-M9, each of which communicates with a nearby base station. The base stations B1-B10 are connected to and controlled by a mobile services switching center (MSC) 20. The MSC 20, in turn, is connected to a central office (not shown in FIG. 1) in the landline (wireline) public switched telephone network (PSTN) or to a similar facility such as an integrated system digital network (ISDN). The MSC switches calls between wireline and mobile subscribers, controls signalling to the mobile stations, compiles billing statistics, stores subscriber service profiles, and provides for the operation, maintenance and testing of the system.
Access to a cellular system by any of the mobile stations M1-M9 is controlled on the basis of a mobile identification number (MIN) and an electronic serial number (ESN) which are stored in the mobile station. The MIN is a digital representation of the 10-digit directory telephone number assigned to each mobile subscriber by the home system operator. The electronic serial number (ESN) is assigned by the manufacturer and permanently stored in the mobile station. The MIN/ESN pair is sent from the mobile station when originating a call and its validity is checked by the MSC. If the MIN/ESN pair is determined to be invalid (e.g., if the ESN has been blacklisted because the mobile station was reported to be stolen), the system may deny access to the mobile station. The MIN is also sent from the system to the mobile station when alerting the mobile station of an incoming call.
When turned on (powered up), each of the mobile stations M1-M9 enters the idle state (standby mode) and tunes to and continuously monitors the strongest control channel (generally, the control channel of the cell in which the mobile station is located at that moment). When moving between cells while in the idle state, the mobile station will eventually "lose" radio connection on the control channel of the "old" cell and tune to the control channel of the "new" cell. The initial tuning to, and the change of, control channel are both accomplished automatically by scanning all the control channels in operation in the cellular system to find the "best" control channel. In the United States, there are 21 "dedicated" control channels (predefined frequencies) in each cellular system which means that the mobile station has to scan a maximum number of 21 RF channels. When a control channel with good reception quality is found, the mobile station remains tuned to this channel until the quality deteriorates again. In this manner, the mobile station remains "in touch" with the system and may receive or initiate a telephone call through one of the base stations B1-B10 which is connected to the MSC 20.
To detect incoming calls, the mobile station continuously monitors the current control channel to determine whether a page message addressed to it (i.e., containing its MIN) has been received. A page message will be sent to the mobile station, for example, when an ordinary (landline) subscriber calls the mobile subscriber. The call is directed from the PSTN to the MSC 20 where the dialed number is analyzed. If the dialed number is validated, the MSC 20 requests some or all of the base stations B1-B10 to page the called mobile station throughout their corresponding cells C1-C10. Each of the base stations B1-B10 which receive the request from the MSC 20 will then transmit over the control channel of the corresponding cell a page message containing the MIN of the called mobile station. Each of the idle mobile stations M1-M9 which is present in that cell will compare the MIN in the page message received over the control channel with the MIN stored in the mobile station. The called mobile station with the matching MIN will automatically transmit a page response over the control channel to the base station which then forwards the page response to the MSC 20. Upon receiving the page response, the MSC 20 selects an available voice channel in the cell from which the page response was received (the MSC maintains an idle channel list for this purpose), and requests the base station in that cell to order the mobile station via the control channel to tune to the selected voice channel. A through-connection is established once the mobile station has tuned to the selected voice channel.
When, on the other hand, a mobile subscriber initiates a call (e.g., by dialing the telephone number of an ordinary subscriber and pressing the "send" button on the telephone handset in the mobile station), the dialed number and MIN/ESN pair for the mobile station are sent over the control channel to the base station and forwarded to the MSC 20 which validates the mobile station, assigns a voice channel and establishes a through-connection for the conversation as described before.
If the mobile station moves between cells while in the conversation state, the MSC 20 will perform a "handoff" of the call from the old base station to the new base station. The MSC 20 selects an available voice channel in the new cell and then orders the old base station to send to the mobile station on the current voice channel in the old cell a handoff message which informs the mobile station to tune to the selected voice channel in the new cell. The handoff message is sent in a "blank and burst" mode which causes a short but hardly noticeable break in the conversation. Upon receipt of the handoff message, the mobile station tunes to the new voice channel and a through-connection is established by the MSC 20 via the new cell. The old voice channel in the old cell is marked idle in the MSC 20 and may be used for another conversation.
The cellular telephone system of FIG. 1 had its origin in the provision of car telephone service. In the last few years, however, there has been an increasing shift towards the use of lightweight pocket telephones in homes, offices, public meeting places, and in virtually any other place the user can obtain service. The next step in this evolution is the emerging concept of "personal communication services" (PCS) or what has sometimes been referred to as services at "walking speeds." The goal is that not only telephone calls but also facsimile, computer data, paging messages and even video signals can be transmitted and received by a user moving around, for example, inside a building, a factory, a warehouse, a shopping mall, a convention center, an airport, or an open area.
PCS systems operate on lower power and use smaller cellular structures than conventional wide area (vehicular) cellular systems so as to provide the high-quality, high-capacity radio coverage needed for private business and other applications. By reducing the transmit power of the base station, the size of the cell (or cell radius) and, with it, the frequency reuse distance are reduced thereby resulting in more channels per geographic area (i.e., increased capacity). Additional benefits of a smaller cell include a longer "talk time" for the user since the mobile station will use substantially lower transmit power than in a larger cell and consequently its battery will not need to be recharged as often.
The cellular industry has grown accustomed to using the terms "macrocell," "microcell," and "picocell" to distinguish the relative size of the cells required for a particular application (indoor or outdoor). The term "macrocell" generally refers to a cell which is comparable in size to cells in a conventional cellular telephone system (e.g., a radius of 1 Km or more). A macrocell usually serves rapidly moving users and covers low to medium usage areas. The terms "microcell" and "picocell," on the other hand, refer to the progressively smaller cells which are used, for example, in a PCS system. A microcell serves the slowly moving users and may cover a public indoor or outdoor area (e.g., a convention center or a busy downtown street). A picocell may cover an office corridor or a floor of a high rise building. Microcells and picocells can also cover high-density pedestrian areas or busy thorough-fares (streets or highways) in a conventional cellular system.
It is now clear that the next generation cellular systems will implement a hierarchial cell structure of macrocells, microcells and picocells which may include one or more private systems (e.g., a wireless PBX system for an office building). From a system (MSC) perspective, the base stations in the microcells and picocells can be viewed as extensions of the base stations in adjoining or overlapping macrocells. In this case, the microcell and picocell base stations may be connected to the macrocell base station via digital transmission lines, for example. Alternatively, the microcells and picocells may be treated just like macrocells and be connected directly to the MSC. From a radio coverage perspective, the macrocells, microcells and picocells may be distinct from each other or, alternatively, overlaid one on top of the other to handle different traffic patterns or radio environments. For example, handoff between microcells may sometimes be difficult to perform around street corners, particularly where the users are moving so rapidly that the signal strength variations are in excess of 20 dB per second. In this situation, it may be possible to use an "umbrella" macrocell for the rapidly moving users and to use microcells for the slowly moving users. By managing different types of users differently in this way, handoff between microcells may be avoided for the rapidly moving users which are subject to the severe street corner effects.
The original cellular radio systems, as described above, used analog transmission methods, specifically frequency modulation (FM), and duplex RF channels in accordance with the Advanced Mobile Phone Service (AMPS) standard. According to the AMPS standard, each control or voice channel between the base station and the mobile station consists of a pair of separate frequencies, a forward (downlink) frequency for transmission by the base station (reception by the mobile station) and a reverse (uplink) frequency for transmission by the mobile station (reception by the base station). The AMPS system, therefore, is a single-channel-per-carrier (SCPC) system allowing for only one voice circuit (telephone conversation) per RF channel. Different users are provided access to the same set of RF channels with each user being assigned a different RF channel (pair of frequencies) in a technique known as frequency division multiple access (FDMA). This AMPS (analog) architecture was the basis for the industry standard sponsored by the Electronics Industries Association (EIA) and the Telecommunication Industry Association (TIA), and known as EIA/TIA-553.
In the late 1980s, however, the cellular industry in the United States began migrating from analog to digital technology, motivated in large part by the need to address the growth in the subscriber population and the increasing demands on system capacity. It was recognized early that the capacity improvements sought for the next generation cellular systems could be achieved by the introduction of microcells or picocells to the specific areas where increased capacity is needed (i.e., cell splitting those areas) or by the use of more advanced digital radio technology which could be applied to the existing macrocells, or by a combination of both approaches. Thus, for example, analog microcells may be implemented to cover "dead spots" (areas where topography or zoning or other restrictions prevent penetration of radio signals) or "hot spots" (areas with heavy localized traffic). In this instance, coverage or capacity may be improved for the existing subscriber base which is using analog mobile stations. However, the actual capacity gain is limited by the use of the analog AMPS technology. The effectiveness of the microcellular (cell splitting) concept in increasing capacity can be maximized only by the use of digital technology (which requires new digital-capable mobile stations).
In the effort to go digital, the EIA/TIA has developed a series of digital standards which rely on voice encoding (digitization and compression) and time division multiple access (TDMA) techniques to multiply the number of voice circuits (conversations) per RF channel (i.e, to increase capacity). The original standard in this series was known as the IS-54 standard. To ease the transition from analog to digital and to allow the continued use of existing analog mobile stations, the IS-54 standard supported the original AMPS analog voice and control channels and additionally provided for the use of digital traffic channels for speech (but not digital control channels) within the existing AMPS network. This "dual-mode" (analog-digital) standard, therefore, became known as the digital AMPS (D-AMPS) standard. More recently, the industry has developed a new specification for D-AMPS which includes a digital control channel suitable for supporting public or private microcell operation, extended mobile station battery life and end-user features characteristic of PCS. This new specification is known as IS-136 and it builds on the IS-54B standard (the current revision of IS-54). All of the foregoing standards are hereby incorporated herein by reference (copies of these standards may be obtained from the Electronics Industries Association; 2001 Pennsylvania Avenue, N.W.; Washington, D.C. 20006).
According to IS-54B and as shown in FIG. 2, each RF channel is time division multiplexed (TDM) into a series of repeating time slots which are grouped into frames carrying from three to six digital speech channels (three to six telephone conversations) depending on the source rate of the speech coder used for each digital speech channel. Each frame on the RF channel comprises six equally sized time slots (1-6) and is 40 ms long (i.e, there are 25 frames per second). The speech coder for each digital traffic channel (DTCH) can operate at either full-rate or half-rate. A full-rate DTCH uses two equally spaced slots of the frame (i.e., slots 1 and 4, or slots 2 and 5, or slots 3 and 6). When operating at full-rate, the RF channel may be assigned to three users (A-C). Thus, for example, user A is assigned to slots 1 and 4, user B is assigned to slots 2 and 5, and user C is assigned to slots 3 and 6 of the frame as shown in FIG. 2. Each half-rate DTCH uses only one time slot of the frame. At half-rate, the RF channel may be assigned to six users (A-F) with each user being assigned to one of the six slots of the frame as also shown in FIG. 2. Thus, it can be seen that the DTCH as specified in the IS-54B standard allows for an increase in capacity of from three to six times that of the analog RF channel. At call set-up or handoff, a dual-mode mobile station will be assigned preferably to a digital traffic channel (DTCH) and, if none is available, it can be assigned to an analog voice channel (AVC). An analog-only mobile station, however, can only be assigned to an AVC.
The IS-136 standard specifies a digital control channel (DCCH) which is defined similarly to the digital traffic channel (DTCH) specified in IS-54B (i.e., on the same set of RF channels and with the same TDMA frame format and slot size). Referring back to FIG. 2, a half-rate DCCH would occupy one slot while a full-rate DCCH would occupy two slots out of the six slots in each 40 ms frame. The DCCH slots may then be mapped into different logical channels which are organized into a series of superframes. FIG. 3 shows the superframe structure of a full-rate DCCH according to IS-136 (in this example, the DCCH is defined over channel "A" in the TDMA frame). A superframe is defined in IS-136 as the collection of 32 consecutive time slots (640 ms) for a full-rate DCCH (16 slots for a half-rate DCCH). The logical channels specified in IS-136 include a broadcast control channel (BCCH) for carrying system-related information which is broadcast to all mobile stations, and a short message service, paging and access response channel (SPACH) for carrying information which is sent to specific mobile stations.
As shown in FIG. 3, the BCCH is divided into logical subchannels each of which is assigned an integer number of DCCH slots. The BCCH subchannels include a fast BCCH (F-BCCH), an extended BCCH (E-BCCH) and a point-to-multipoint short message service BCCH (S-BCCH). The F-BCCH is used to broadcast DCCH structure parameters and other parameters required for accessing the system (the first slot in a superframe is always assigned to the F-BCCH). The E-BCCH is used to broadcast information that is not as time-critical (for the operation of the mobile stations) as the information in the F-BCCH. The S-BCCH is used for the broadcast short message service (SMS) which can deliver alphanumeric messages of common interest to all mobile stations (e.g., traffic reports). The SPACH is also divided into logical subchannels each of which is assigned a given number of time slots on a fully dynamic basis (and, thus, these subchannels are not explicitly shown in FIG. 3). The SPACH subchannels include a point-to-point short message service channel (SMSCH), a paging channel (PCH) and an access response channel (ARCH). The SMSCH is used for carrying alphanumeric messages of interest to a specific mobile station (e.g., stock quotations). The PCH is used for carrying paging messages to different mobile stations (each mobile station is assigned to a predefined "paging frame class" which defines the periodicity with which it reads the PCH). The ARCH is used for responding to access requests from one of the mobile station (e.g., by delivering a channel assignment message to this mobile station).
An idle mobile station operating on the DCCH of FIG. 3 need only be "awake" (monitoring) during certain time slots (e.g., the F-BCCH or its assigned PCH slot) in each DCCH superframe and can enter "sleep mode" at all other times. While in sleep mode, the mobile station turns off most internal circuitry and saves battery power. Sleep mode operation reduces battery drain in the mobile station during idle mode and increases "talk time" for the user (i.e., the period between battery recharging). Furthermore, the user of this mobile station may be able to access a host of value-added data services through the SMS facilities provided in IS-136. For example, the user could receive frequent traffic updates over the S-BCCH. The user could also retrieve, for example, desired stock quotations over the SMSCH. These examples and a variety of other ISDN-type services are facilitated by the use of the DCCH specified in IS-136. In addition, IS-136 provides a cell selection procedure for hierarchial cell structures which optimizes capacity by biasing the cell selection criteria in favor of microcell selection. The specification also provides substantial support for defining private or residential systems through the assignment of unique identities to these systems which are different from the identity of the local system operator.
Because of the inherent advantages associated with IS-136 operation, it is desirable to have IS-136 compliant mobile stations access the system via the DCCH. However, unlike the analog control channel which is defined within a fixed frequency range (21 dedicated RF channels), the DCCH can be assigned to any frequency allocated to the system and, depending on capacity requirements, one or more additional DCCHs may be defined on other RF channels in the system. Hence, a DCCH-capable mobile station cannot simply scan a fixed set of RF channels in order to locate and lock onto a DCCH for the purpose of obtaining service, but must find the DCCH through some other method. One method for locating a DCCH is for the mobile station to scan a list of the most recent valid DCCHs to which the mobile station has successfully locked. Another method, which is specified in IS-136, is for the mobile station to scan a set of "probability blocks" containing lists of RF channels where it is likely to find a DCCH. In certain situations, however, these scanning methods may require a relatively long time to locate an operational DCCH. Hence, an alternative method according to IS-136 is to transmit to the mobile station a DCCH locator message on the analog control channel (or on the DTCH). This message informs the mobile station of where to go in order to find a DCCH (e.g., identifies the RF channel number and slot number for the DCCH).
The foregoing methods for locating a DCCH do not allow for a great degree of flexibility in directing a mobile station to the most appropriate DCCH for purposes of providing service to this mobile station. In particular, none of these methods takes into account the geographic location or service preference of the mobile subscriber. Furthermore, the method of broadcasting a DCCH locator message on the analog control channel has proven to be problematic in practice. At present, the industry is bringing to the market dual-mode mobile stations which are capable of operating in either an analog mode compliant with the older EIA/TIA-553 (AMPS) standard or a digital mode compliant with the newer IS-136 (D-AMPS) standard. However, there are currently over 5 million analog mobile stations in the field which were designed for operation on the analog control channel as originally specified in the AMPS standard. Many of these analog mobile stations are unable to process the DCCH locator message which can be sent on the analog control channel as specified in IS-136. When this message is sent, these mobile stations lose synchronization on the current control channel and, therefore, rescan for a valid control channel. If this message is sent frequently, these mobile stations go into a permanent "No Service" state. Even when this message is sent less frequently, these analog mobile stations are likely to miss incoming calls or fail to originate calls at a failure rate which is higher than normal.
The present invention overcomes the limitations of the prior DCCH locating methods and the problem associated with the processing of the DCCH locator message by existing analog mobile stations. With the present invention, the system is able to dynamically point DCCH-capable mobile stations to a DCCH which best suits the objectives of the user or the system. From a user service perspective, it may be desirable to steer the mobile station to a DCCH in its preferred private system whenever the mobile station is nearby so that the user can take advantage of any services provided exclusively by that private system. From a system capacity standpoint, it may be desirable to direct the mobile station to a DCCH in the smallest cell (e.g., microcell) in the area. Furthermore, from a performance point of view, it may be desirable to serve the mobile station from the cell in the hierarchial cell structure which provides the best radio coverage in his location. These objectives and others can be met by use of the present invention.